LEDs have been proposed for use as light sources for many years. Recent developments of LED technology have expanded the use of LEDs from signs and message boards to automobile interior and exterior lights and traffic lights. However, the light output of LED light sources is very sensitive to temperature, and in fact is permanently degraded by excessive temperature. The aforementioned applications require that the substrate for the LEDs has a high heat dissipation capability, good heat-resistant property, and high mechanical strength. An LED light source with high heat dissipation capability to quickly carry away excessive heat is essential for maintaining its high performance.
A light source for illuminating an information source is often required in many applications, such as liquid crystal displays (LCDs). In general, a backlight module is required for the LCDs to illuminate the information to be displayed. LEDs are moving into the LCD backlight module market as well. The advantages of LED light sources include long life, ease of replacement, robust mechanical property, relatively high efficacy, and better color quality than fluorescent lamps.
Certain applications (e.g., avionics) require a specific chromaticity of light emitted from the LCD backlight module. However, most commercially available LEDs are made with a limited number of chromaticity choices and their chromaticity may change over time. An LED light source with a raised LED, as shown in FIG. 1, to improve the chromaticity of a combined light was disclosed in U.S. Pat. No. 6,666,567. The raised structure permits lights to be emitted from the base of the LED. Additionally, reflective protrusions may be placed beneath the raised LED to aid in redirecting the light trajectory. A combination of fluorescent lamps and LEDs were also proposed to form a hybrid light source. However, all these schemes increase the complexity and cost of the light source.
An LCD backlight which includes a first LED array that provides light with a first chromaticity and a second LED array that provides light with a second chromaticity, was disclosed in another U.S. Pat. No. 6,608,614. The lights emitted from these two LED arrays are combined through a combining element (e.g., a wave guide) and then projected towards an LCD stack. The LED chip normally emits lights in a direction that is approximately perpendicular to the chip surface. The directions of the lights emitted from the first and the second LED arrays are approximately perpendicular and parallel to the panel surface, respectively. A separate combining element is required in this light source. The chromaticity of the combined light can only be adjusted by changing the chromaticity of the second LED array through a control system. Therefore, there is a limited flexibility for chromaticity adjustment.
According to another prior art, a Luxeon side-emitter having packaged LED chips was disclosed, as shown in FIG. 2. The side-emitter may provide good uniformity of combined lights but the light intensity is poor. In addition, these packaged LED chips normally occupy a large area. It is known that the majority of lights emitted from LED chips travel in a direction approximately perpendicular to the chip surface. Therefore, the LED chips need to be arranged in a way such that the lights emitted from different LED chips have a chance to be combined and mixed in order to achieve desired chromaticity before they reach a display screen.
Surface mountable LEDs can achieve the side-emitting characteristics by attaching the LED chips in a way that allows the emitted lights to radiate approximately parallel to the mounting surface. Then, the lights can be further combined to obtain a desired chromaticity and redirected to the display screen. The packaging of SMD LEDs consists of circuit type and lead-frame type. The lead-frame type uses metallic lead frames as substrates and injection or compression molding, followed by cutting the structure into SMD LED packages as shown in FIG. 3. The circuit type uses composite circuit board as substrate, followed by compression molding and cutting the structure into SMD LED packages as shown in FIG. 4. The above two SMD LED packages can be found in U.S. Pat. Nos. 6,573,580 and 6,740,903. Another SMD LED package using silicon wafer as the substrate is proposed in U.S. Pat. No. 6,531,328 in 2003. This manufacturing process is not mature and the fragile silicon wafer may aggravate the poor manufacturing yield.